Re: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian

From: Daniel Borkmann <daniel@iogearbox.net>
To: Martin KaFai Lau <kafai@fb.com>, Okash Khawaja <osk@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>, Yonghong Song <yhs@fb.com>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	"David S. Miller" <davem@davemloft.net>,
	netdev@vger.kernel.org, kernel-team@fb.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian
Date: Tue, 10 Jul 2018 10:21:02 +0200	[thread overview]
Message-ID: <58136182-0eb1-78c9-ceb9-402418c7d10c@iogearbox.net> (raw)
In-Reply-To: <20180709183236.r4b7gzmev5h4lcbw@kafai-mbp.dhcp.thefacebook.com>

On 07/09/2018 08:32 PM, Martin KaFai Lau wrote:
> On Sun, Jul 08, 2018 at 05:22:03PM -0700, Okash Khawaja wrote:
>> When extracting bitfield from a number, btf_int_bits_seq_show() builds
>> a mask and accesses least significant byte of the number in a way
>> specific to little-endian. This patch fixes that by checking endianness
>> of the machine and then shifting left and right the unneeded bits.
>>
>> Thanks to Martin Lau for the help in navigating potential pitfalls when
>> dealing with endianess and for the final solution.
>>
>> Fixes: b00b8daec828 ("bpf: btf: Add pretty print capability for data with BTF type info")
>> Signed-off-by: Okash Khawaja <osk@fb.com>
>>
>> ---
>>  kernel/bpf/btf.c |   32 +++++++++++++++-----------------
>>  1 file changed, 15 insertions(+), 17 deletions(-)
>>
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -162,6 +162,8 @@
>>  #define BITS_ROUNDDOWN_BYTES(bits) ((bits) >> 3)
>>  #define BITS_ROUNDUP_BYTES(bits) \
>>  	(BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))
>> +const int one = 1;
>> +#define is_big_endian() ((*(char *)&one) == 0)

Also here, in the kernel archs provide proper definitions.

>>  #define BTF_INFO_MASK 0x0f00ffff
>>  #define BTF_INT_MASK 0x0fffffff
>> @@ -991,16 +993,13 @@ static void btf_int_bits_seq_show(const
>>  				  void *data, u8 bits_offset,
>>  				  struct seq_file *m)
>>  {
>> +	u8 left_shift_bits, right_shift_bits;
> Nit.
> Although only max 64 bit int is allowed now (ensured by btf_int_check_meta),
> it is better to use u16 such that it will be consistent to BTF_INT_BITS.
> 
>>  	u32 int_data = btf_type_int(t);
>>  	u16 nr_bits = BTF_INT_BITS(int_data);
>>  	u16 total_bits_offset;
>>  	u16 nr_copy_bytes;
>>  	u16 nr_copy_bits;
>> -	u8 nr_upper_bits;
>> -	union {
>> -		u64 u64_num;
>> -		u8  u8_nums[8];
>> -	} print_num;
>> +	u64 print_num;
>>  
>>  	total_bits_offset = bits_offset + BTF_INT_OFFSET(int_data);
>>  	data += BITS_ROUNDDOWN_BYTES(total_bits_offset);
>> @@ -1008,21 +1007,20 @@ static void btf_int_bits_seq_show(const
>>  	nr_copy_bits = nr_bits + bits_offset;
>>  	nr_copy_bytes = BITS_ROUNDUP_BYTES(nr_copy_bits);
>>  
>> -	print_num.u64_num = 0;
>> -	memcpy(&print_num.u64_num, data, nr_copy_bytes);
>> -
>> -	/* Ditch the higher order bits */
>> -	nr_upper_bits = BITS_PER_BYTE_MASKED(nr_copy_bits);
>> -	if (nr_upper_bits) {
>> -		/* We need to mask out some bits of the upper byte. */
>> -		u8 mask = (1 << nr_upper_bits) - 1;
>> -
>> -		print_num.u8_nums[nr_copy_bytes - 1] &= mask;
>> +	print_num = 0;
>> +	memcpy(&print_num, data, nr_copy_bytes);
>> +	if (is_big_endian()) {
>> +		left_shift_bits = bits_offset;
>> +		right_shift_bits = BITS_PER_U64 - nr_bits;
>> +	} else {
>> +		left_shift_bits = BITS_PER_U64 - nr_copy_bits;
>> +		right_shift_bits = BITS_PER_U64 - nr_bits;
> Nit.
> right_shift_bits is the same for both cases.  Lets simplify it.
> 
>>  	}
>>  
>> -	print_num.u64_num >>= bits_offset;
>> +	print_num <<= left_shift_bits;
>> +	print_num >>= right_shift_bits;
>>  
>> -	seq_printf(m, "0x%llx", print_num.u64_num);
>> +	seq_printf(m, "0x%llx", print_num);
>>  }
>>  
>>  static void btf_int_seq_show(const struct btf *btf, const struct btf_type *t,
>>